1. Field of the Invention
The present invention relates to an in-plane switching (IPS) mode active matrix liquid crystal display unit.
2. Description of the Related Art
Japanese Patent Publication 2005-70747 (referred to as Document 1) discloses a liquid crystal display unit, which can be classified into one of IPS-mode display units. The prior art display unit comprises a lower structure and an upper structure and accommodates a liquid crystal layer between the upper and lower structures. The whole unit is illuminated with backlight provided below the lower structure. The lower structure consists of a transparent substrate on which a transparent common electrode (first ITO electrode) is formed and coated with an insulating layer. For each pixel, a data line is provided on the insulating layer and coated with a protection layer, on which transparent parallel pixel electrodes (second ITO electrodes) are provided. However, the data lines are not shielded from the liquid crystal layer, the electric fields of the data lines adversely affect the orientation of the liquid crystals and cause portions of the backlight that pass through the edges of the data line to leak to the viewer side. For shielding such undesired leaking light, the upper structure is provided with a black matrix. Since the black matrix occupies a relatively large space, the aperture ratio of the prior art LCD unit is not satisfactory.
Japanese Patent Publication 2002-258321 (referred to as Document 2) discloses an IPS-mode LCD unit in which a lower structure and an upper structure are provided for accommodating a liquid crystal layer and the whole unit is illuminated with backlight in a manner similar to Document 1. The lower structure consists of a transparent substrate, transparent pixel electrodes and data lines, which are formed above the transparent substrate. The pixel electrodes and the data lines are coated with a protection layer on which common electrodes are provided. For each pixel, each common electrode comprises an inner electrode positioned inside of the pixel aperture and an outer edge portion, which overlies the corresponding data line and is raised from the protection layer by an insulator.
Although the data lines of Document 2 are covered with the outer edge portions of the common electrodes, the data lines are still not satisfactorily shielded from the liquid crystal layer. Therefore, liquid crystals are still adversely affected in their orientation by the stray field components of the data lines. Additionally, since transversal field component that contributes to the in-plane switching of liquid crystal is depleted in the area immediately above each common electrode, light transmissivity in this area is low, resulting in a low controllability of light transmissivity. Further, high precision technique is required to control the width of each common electrode because it noticeably affects on the electric field that contributes to the in-plane switching.